Possibly a later myth; the saying predates Newton - also the perceived slight was actually against Hooke's supposed curved spine rather than his height I think.
"We are like dwarfs on the shoulders of giants, so that we can see more than they, and things at a greater distance, not by virtue of any sharpness of sight on our part, or any physical distinction, but because we are carried high and raised up by their giant size." -- John of Salisbury, The Metalogicon (1159)
https://www.oxfordreference.com/display/10.1093/acref/978019...
It is apparently in one of his letters - to Hooke.